Time domain magnetization dynamics study to estimate interlayer exchange coupling constant in Nd-Fe-B/Ni₈₀Fe₂₀ films

Interlayer exchange coupling (J_ex) between a hard magnetic Nd-Fe-B layer and a soft magnetic Ni₈₀Fe₂₀ layer is studied by means of time-resolved magneto-optical Kerr effect (TRMOKE) microscope. Whereas a single 16 nm thick Nd-Fe-B layer without Ni₈₀Fe₂₀ showed high coercivity of μ₀H_c = 2 T and resonance frequency of f_r = 161 GHz at external bias magnetic field of μ₀H_b = 2 T due to the high anisotropy field, those of the bi-layer Nd-Fe-B (16 nm)/Ni₈₀Fe₂₀ (5 nm) are dramatically reduced to μ₀H_c = 1.34 T and f_r = 74.4 GHz. When the Nd-Fe-B and Ni₈₀Fe₂₀ are separated by a 1 nm thick non-magnetic Mo layer, by contrast, the coercivity recovered partially to μ₀H_c = 1.9 T but the frequency further reduced to f_r = 63.4 GHz. We derived J_ex based on a simple macrospin model, whose value reduced from 3.9 ± 0.1 mJ/m² for the bi-layer without the Mo layer to 0.1 ± 0.1 mJ/m² with the Mo layer. The reduction in J_ex suggested that the interlayer exchange decoupling between the Nd-Fe-B and the Ni₈₀Fe₂₀ layers was responsible to the recovery of μ₀H_c and the reduction of f_r by the insertion of the non-magnetic layer. We successfully estimated the interlayer exchange coupling constant in the hard/soft magnetic bilayer system by TRMOKE and macrospin-modeling, which had been previously difficult because of its high anisotropy and high coercivity. This method is applicable also to the quantitative estimation of the intergranular exchange coupling.